Field of the Invention
This invention relates to an optical system for making a uniform intensity beam with a striped, rectangular section (Q×w: Q>>w). High power laser beams are utilized for welding, cutting, boring, annealing or other heat processing of objects. Necessary wavelengths of lasers depend upon the objects and the kind of the processing. CO2 laser beams of wavelengths of 9 μm to 11 μm are suitable for welding and annealing metal. YAG laser beams of a 1.06 μm wavelength is effective for piercing and annealing. A 532 nm wavelength of the second harmonic generation YAG (SHG-YAG) laser is promising for annealing of amorphous silicon (a—Si). When heat power of laser beams is used, non-uniform intensity beams are sometimes available. However, other heat processing sometimes requires uniform power density of irradiation laser beams.
This application claims the priority of Japanese Patent Application No. 2004-66616 filed on Mar. 10, 2004, which is incorporated herein by reference.
Some processing requires an in-phase beam which means that wave components of light have a common phase on a plane vertical to a beam axis. A beam which has the parallelism property and the in-phase property can propagate afterward without loosing the uniformity of power density. Such an in-phase parallel uniform power density beam is an ideal beam. It is, however, difficult to produce such an in-phase, parallel, uniform beam. Thus, some heat processing adheres only to the uniform power density at a certain plane for reducing the difficulty of endowing a beam with parallelism, in-phase property and uniformity. The plane on which the components have a common phase is called an image plane. If the temporarily uniform beam propagates further over the image plane, the uniformity is disturbed. But an object can be irradiated with the temporarily uniform beam with uniform power density by positioning the object just on the image plane. The present invention tries to propose an optical system for making such a temporal uniform power density beam which has uniform power only on a definite image plane.
A beam emitted from a laser is a cylindrical, parallel beam. Power density of the laser beam is not uniform. The laser beam has been produced in a resonator cavity formed between two spherical concave lenses with large curvature radii. The laser beam has higher power density at the center and lower power density at the periphery. The power distribution of laser beams is ideally a Gaussian distribution. When an object is directly irradiated with an inherent laser beam, the power distribution of the irradiation beam on the object is Gaussian. Some objects and some processings deny Gaussian beams but require uniform power beams. Preparation of uniform power beams requires an optical system for converting a Gaussian laser beam into a uniform power distribution beam. The uniform distribution beam is sometimes called a “tophat” beam, since the power distribution is constant within a definite area and falls to zero outside of the range. The optical system for converting the Gaussian beam into the uniform power density beam is called a “homogenizer”.
There are two methods for converting the Gaussian beam into the uniform power beam. One method is a lens type one relying upon a set of aspherical lenses. The other is a diffraction type one making use of a set of diffraction optical elements (DOEs).
A lens type homogenizer is first described. A typical lens homogenizer consists of two lenses for refracting a cylindrical Gaussian laser beam into a tophat beam. One lens is an intensity-conversion lens which makes uniform distribution of power by enlarging central parts of the Gaussian beam and shrinking peripheral parts of the Gaussian. No ordinary spherical lens has such contradictory functions of partial enlargement and partial reduction. An aspherical lens should be assigned to be the intensity-conversion lens. An input laser beam is converted into a uniform power beam but phases of wave components are disturbed. The output beam is an out-of-phase tophat beam. Here, “out-of-phase” means a state in which wave components have different phases on any planes vertical to an axis. The intensity-conversion lens makes a tophat beam at a sacrifice of in-phase property.
The other lens, which is called a “phase-compensation lens”, equalizes phases of beams. Namely, the phase-compensation lens revives an in-phase state in which all wave components have a common phase on any plane vertical to the beam axis. A pair of the intensity-conversion lens and the phase-compensation lens is called a “lens homogenizer”. The laser power equalization by the pair of aspherical lenses has advantages of low loss, in-phase property and parallelism. The pair of the intensity-conversion lens and the phase-compensation lens is called a “lens homogenizer”.
In addition to the lens homogenizer, there is another type of homogenizer based upon diffraction optical elements (DOEs). The DOE homogenizer makes a uniform or quasi-uniform power density beam by diffraction instead of refraction. FIG. 1 shows an example of the DOE homogenizer which converts a round Gaussian laser beam into a round uniform or quasi-uniform power density beam. A laser (not shown) emits a round Gaussian beam 2. The Gaussian power distribution is shown below on a left side. The Gaussian laser beam 2 goes into a DOE 3. The DOE 3 is a transparent plate having plenty of microelements (pixels) of different height (thickness) steps aligning lengthwise and crosswise. The DOE 3 has a function of diffracting light. A reducing beam 4 diffracted by the DOE 3, which is a non-parallel beam, becomes a tophat beam (uniform density beam) 6 just on an image plane 5. The tophat power distribution of the diffracted beam 6 is shown below on a right side in FIG. 1.
In many cases, sections of the uniform-power diffracted beams 6 formed on the image plane 5 are circles. However, uniform power beams with elliptical sections can be also made on the image plane. A square sectioned (q×q) uniform (tophat) diffraction beam can be also made on the image plane by a DOE. A rectangle sectioned (q×w) uniform (tophat) diffraction beam can be also made on the image plane by another DOE. The word “tophat” means that the beam has uniform power distribution within a definite, restricted region but does not mean that the beam has a circular section.
A wavelength of a laser beam is denoted by “λ”. A DOE has many pixels with a plurality of steps of thickness. A total difference ΔD between the thickest part and the thinnest part of the DOE should coincide with a single wavelength λ. Namely, the total step difference ΔD is given by an equation λ=(n−1)ΔD. A unit of steps is determined to be a quotient of ΔD divided by 2k (k: integer). For example, for k=8 and 2k=256, a step unit is ΔD/256. Step heights of pixels are determined to be multiples of the step unit.
In the example shown in FIG. 1, the diameter of the round uniform density beam on the image plane is smaller than a diameter of an input laser beam. For instance, the diameter of the input laser beam is 2 mm (p and a diameter of an output uniform beam on the image is 1 mmφ. The beam diameter is reduced. Why such a shrinking optics is employed is that the power density of the laser beam is still weak. And required uniform power density for heat-processings should be far strong. The power density should be raised for compensating for imbalance by shrinking the beam diameter of the beam. The DOE produces the uniform power density (tophat) beam 6 on the image plane 5 by diffracting peripheral laser rays inward stronger and central laser rays inward weaker. Although the diffracted beam is uniform just on the image plane, the beam becomes non-uniform at other spots before or after the image plane.
FIG. 1 is only a schematic view of the DOE. The DOE makes use of not refraction light but diffraction light. A variety of noise accompanies the DOE. The DOE utilizes only the first order diffraction light. Second order, third order or other higher order diffraction light or 0-th order light are yielded by the DOE. The higher order components and the 0-th order component are not simply converged as shown in FIG. 1. Thus, the higher order and 0-th order components are noise. This is a weak point of DOEs. However, the noise is negligibly weak in ordinary cases of DOEs. Further drawback is the difficulty of processing caused by a variety of step heights of microelements (pixels). The degree of freedom is large for DOEs. But, the high degree of freedom increases the difficulty of processing and finishing. The high degree of freedom is a drawback of DOEs. Since DOEs are designed for making uniform power density only on an image plane, the uniformity of power density is rapidly degraded at spots deviating forward or backward from the image plane.
The degree of freedom of design is high. The high degree of freedom prevents a designer from determining a single preferable DOE system for making a uniform power distribution beam. There are many probable DOE system candidates capable of preparing a uniform power beam. If a designer selects one among the probable DOE candidates, the chosen one is not always the best one. Unlike the lens type homogenizers, the DOE type homogenizers are plagued by many inherent disadvantages.
The above-described lens type homogenizer (intensity-conversion lens & phase-compensation lens) can make a parallel, in-phase tophat beam. It is no matter for the lens type homogenizer whether the image plane deviates forward or backward from a determined position due to the parallelism and the in-phase property. Furthermore, the lens type homogenizer, which makes use of only refraction, is immune from problems of diffraction, e.g., higher order diffraction and 0-th order diffraction. The lens type homogenizer can produce a neat tophat beam on an image plane. The paired lens homogenizer is an excellent one. Since a lens has inherently a rotation symmetry, the lens homogenizer is quite suitable for producing a cylindrical, circular-sectioned uniform power beam. On the contrary, the lens homogenizer is incompetent to make a cylindrically-asymmetric, e.g., rectangular-, square- or ellipse-sectioned beam.
Laser heat treatments do not always require rotationally-symmetric cylindrical uniform power beams. The desired sections of beams depend upon the kind of heat treatments and the shape of objects. Sometimes a square section q×q of a uniform power density beam is required. Sometimes a rectangle section q×w of a uniform power beam is required. Otherwise, a stripe section q×w (w<<q) may be required for a uniform power density beam. Lens homogenizers are incompetent for the purpose of preparing non-circle beams.
The before-described diffraction optical elements (DOEs) are suitable for the purpose of making uniform power density non-circular (rectangular, striped) sectioned beams. A DOE is an optical device having plenty of tiny units (pixels) aligning crosswise and lengthwise in two dimensions, having different thicknesses (heights), and diffracting light into light with an arbitrary direction, shape and density distribution. A DOE has extremely high degree of freedom which is a product of the pixel number and the height step number. The high degree of freedom enables a DOE to make a q×q square sectioned uniform beam or a q×w rectangle sectioned uniform beam. DOE homogenizers are competent for such a purpose of making square, rectangular sectioned beams.
A long stripe-sectioned (Q×w; Q>>w) uniform density beam as a limit of high Q/w rate rectangles is sometimes required. For example, in the case of producing large liquid crystal display panels, such a long stripe uniform power laser beam is requested. The liquid crystal display panel has plenty of thin film transistors made by evaporating amorphous silicon (a—Si) films on a glass substrate and processing the amorphous silicon films to transistor devices. For some purposes, amorphous silicon transistors are available. But, electron mobility on amorphous silicon is not high enough yet for producing liquid crystal display panels of high quality.
The electron mobility can be enhanced by heating the amorphous silicon (a—Si), enlarging crystalline granules, and converting the amorphous silicon into polycrystalline silicon (poly-Si). But, it is undesirable to heat the amorphous silicon film on the glass substrate. If the amorphous silicon is heated by a heater, the glass substrate is also heated together. For example, it is preferable to heat the amorphous silicon at temperatures between 800° C. and 1000° C. for converting a—Si to poly-Si. Ordinary glass substrates have weak resistance against heat. If the glass substrate is heated up at 800° C., the glass substrate is softened and melted out. Direct heating is unsuitable. What is necessary is a special means which can heat only the amorphous silicon without heating the substrate glass. Neither resistor heaters nor induction heaters are competent. They need much time for heating the a—Si film. In the meantime, heat is conducted to heat-fragile grass substrates. A single candidate may be rapid heating by momentary light irradiation. If light carried a great amount of heat only to the amorphous silicon film without heating the substrate, the light heating would perhaps succeed in converting a—Si into poly-Si without melting the under-substrate.
Momentary irradiation of large power lasers should be a favorable candidate. In the case, the laser light should have a short wavelength which amorphous silicon effectively absorbs. Infrared light and red-yellow visible light are not fully absorbed by amorphous silicon. Ultraviolet rays are desirable, since amorphous silicon can absorb the ultraviolet rays at high efficiency. Excimer lasers, for example, KrF lasers or ArF lasers, can produce high power ultraviolet rays, which satisfy the condition imposed on wavelengths and power. But, the ultraviolet rays made by the KrF lasers or ArF lasers are still plagued by disorder of wavefronts and irregular, non-Gaussian power distribution. The KrF or ArF lasers are disqualified. A YAG laser emits a large power 1.06 μm Gaussian beam. But, 1.06 μm is infrared light which passes through amorphous silicon without loss. The YAG laser is incompetent. An SHG-YAG laser produces a 532 nm light beam. Amorphous silicon can absorb 532 nm. Thus, SHG-YAG lasers are effective candidates for a light source of the high power heating treatment. The SHG-YAG lasers on sale are still annoyed with shortage of power. However, large power SHG-YAG lasers will be made in near future.
In the case of liquid crystal display panels, a small diameter round tophat beam is unuseful, since an object is to momentarily heat a wide a—Si film piled upon a wide grass substrate. Scanning a wide glass with a spot beam many times lengthwise and crosswise would take too long time, which would reduce throughput. A long stripe-sectioned uniform power beam is far favorable than the spot tophat beam. What is desired is a strong, uniform power beam with a section of stripe (Q×w; Q>>w). Q is a length and w is a depth. The beam should be scanned in a direction parallel with the thickness (depth) w. The size of an object is denoted by K×H, where K is a length and H is a depth. The object can be fully scanned by K/Q times and H length of scanning of the Q long stripe uniform beam. A longer length Q of the scanning beam reduces scanning times of K/Q.
Otherwise, there are some utilities of requiring a strong, stripe-sectioned uniform density beam in addition to the conversion form a—Si to poly-Si on liquid crystal display panels. In these cases, an ideal beam would be a parallel, in-phase (coherent), uniform, stripe-section beam. However, satisfying parallelism, in-phase property, uniformity and stripe section is very difficult. Thus, we give up parallelism and in-phase property. What is essential is uniformity and stripe section just on a definite image plane. The present invention relates to a DOE homogenizer for preparing a strong, uniform power density, zone-section beam for light heating treatment.